Magnetic flux may exist in “free-space,” in materials that have the magnetic characteristics of free-space, and in materials with magnetically conductive characteristics. The degree of magnetic conduction in magnetically conductive materials is typically indicated with a B-H hysteresis curve, by a magnetization curve, or both.
Permanent magnets may now be composed of materials that have a high coercively (Hc), a high magnetic flux density (Br), a high magneto motive force (mmf), a high maximum energy product (BHmax), with no significant deterioration of magnetic strength over time. An example is the N52 NdFeB permanent magnet from magnet supplier, www.magnet4sale.com, which has an Hc of 1,079,000 Amperes/meter, a Br of 1.427 Tesla, an mmf ranging up to 575,000 Ampere-turns, and a BHmax of 392,000 Joules/meter3.
According to Moskowitz, “Permanent Magnet Design and Application Handbook” 1995, page 52, magnetic flux may be thought of as flux lines which always leave and enter the surfaces of ferromagnetic materials at right angles, which never can make true right-angle turns, which travel only in straight or curved paths, which follow the shortest distance, and which follow the path of lowest reluctance (resistance to magneto motive force).
Free space presents a high reluctance path to magnetic flux. There are many materials that have the magnetic characteristics similar to those of free space. There are other materials that offer a low or lower reluctance path for magnetic flux, and it is these materials that typically comprise a defined and controllable magnetic path.
High-performance magnetic materials for use as magnetic paths within a magnetic circuit are now available and are well suited for the (rapid) switching of magnetic flux with a minimum of eddy currents. Certain of these materials are highly nonlinear and respond to a “small” applied magneto motive force (mmf) with a robust generation of magnetic flux (B) within the material. The magnetization curves of such materials show a high relative permeability (ur) until the “knee of the curve” is reached, at which point ur decreases rapidly approaching unity as magnetic saturation (Bs) is reached.
A “reluctance switch” is a device or means that can significantly increase or decrease the reluctance of a magnetic path. This is ideally done in a direct and rapid manner, while allowing a subsequent restoration to the previous reluctance, also in a direct and rapid manner. A reluctance switch typically has analog characteristics. By way of contrast, an off/on electric switch typically has a digital characteristic, as there is no electricity “bleed-through.” With the current state of the art, however, reluctance switches exhibit some magnetic flux bleed-through. Reluctance switches may be implemented mechanically, such as to cause keeper movement to create an air gap, or rotating a lower reluctance material through an air gap (a high reluctance path segment) or electrically by various other means.
One electrical reluctance switch implementation uses a control coil or coils wound around a magnetic path or a sub-member that affects the path. U.S. Navy publication, “Navy Electricity and Electronics Series, Module 8—Introduction to Amplifiers” September 1998, page 3-64 to 3-66 describes how to modulate alternating current by changing the reluctance of the entire primary magnetic path by these means, one of which is used in a saturable-core reactor and the other in a magnetic amplifier. Flynn, U.S. Pat. No. 6,246,561; Patrick et al., U.S. Pat. No. 6,362,718; Pedersen, U.S. Pat. No. 6,946,938; Marshall, and US Patent Application 2005/01256702-A1 all disclose methods and apparatus that employ this type of reluctance switch for switching magnetic flux from a stationary permanent magnet or magnets for the purpose of generating electricity (and/or motive force).
Another electrical means of implementing a reluctance switch is the placement within the primary magnetic path of certain classes of materials that change (typically increase) their reluctance upon the application of electricity. A different way of implementing a reluctance switch is to saturate a sub-region of a primary magnetic path by inserting conducting electrical wires into the material comprising the primary magnetic path. Such a technique is described by Konrad and Brudny in “An Improved Method for Virtual Air Gap Length Computation,” in IEEE Transactions on Magnetics, Vol. 41, No. 10, October 2005. A further electrical means of implementing a reluctance switch is described by Valeri Ivanov of Bulgaria on the website www.inkomp-delta.com.